Voodoo Lab Overdrive mods and tweaks.

[Plexi]

Nice clean OD...but, too much clean to my taste.


I was looking for some mod to increase gain and distortion... and found this great mod of Mark:


Great job, as always, Mark!

I only reduce (put a b50k pot instead, limited to 10k) the 47k res from 2.2uF to ground in the IC2 stage of the Voodoo Lab, and voilà: more volume, more gain and presence.
I would try to add some clipping diodes there...

From pins 6 to 7:
- Reduce the 150k = more gain?
- Reduce the 220pf = more compresion?
- Reduce the 2.2uF cap = more highs?

[antonis]

1. No..

2. More bass will be amplified..

3. Less bass will be amplified..


Lowering R8 leads to lower Gain (R8/R9)

C5 & C6 "counterfight" each other.. 
C5 bypasses frequencies out of NFB loop (shunts R_F) whereas C6 admits frequencies into NFB loop (in series with R_G).. 

[amz-fx]

Here is what I would suggest as a start:

Reduce R5 to 4.7k
Increase C3 to 0.047
Change gain pot to 100k-RA

Make those changes and see how it sounds before deciding on more changes. 

regards, Jack

[Plexi]

Thanks, guys!

Las night I reduced R5 and R9 to 10k, and increase gain (and volume).
The trick is there: but! I sugest to add some hard clipping (4148) where Mark placed on ICB stage.
That change it to another kind of (distortion?) sound.

I'll keep trying some values on Gain pot, I found the perfect balance b/clean and distortion with A250K.

[Kennt82]

Lower R5 for sure!   Collect all the MXR Dist+, DOD250 and OCD mods you can find and you'll see most can apply.

[nonoxxx]

- Reduce the 150k = more gain?  : the gain is calculated by this formula : 1+ 150k / 47 k   so reducing the 150 k is like reducing the gain of the second stage.

- Reduce the 220pf = more compresion?    : these little caps in te loop are here to make a low pass , so increasing the value will cut more high, reducing will cut less  highs.

- Reduce the 2.2uF cap = more highs?    : no,  you will have less bass for the output buffer

[Plexi]

Thanks Kennt and nonoxxx!

I take a look to their 'son'; the OCD, to figure out some tweaks... but I guess I'm killing all their nature.
So..I'll limit all the tweaks to C5 to get a bit more grit.

...and, leave the OCD, Dist+ and Grey DOD for later 

[Mark Hammer]

1) I second Jack's suggest changes to the original VL circuit.  In fact, it may be the case that what you have shown is erroneous (a maximum gain of 11x for the clipping stage of an overdrive pedal? )  The only change I would make from his suggestions is that I think he meant for C3 to be 470nf and not 47nf.

2) The Roseyray schematic shown is not the final version.  In the version shown there is a DC path from the output of stage 2 back to its input.  Trace a path from its output back to the non-inverting input of that stage and you'll see that nowhere does the signal HAVE to pass through a cap.  It has to pass through one on the way out, via the volume pot, but that potential feedback path is trouble.

I fixed it by inserting an electrolytic cap  just before the 6k8 fixed resistor and on the output of the second stage, just ahead of the midscoop filter.  1uf-2u2 ought to do the trick.

3) The Roseyray is sort of a 2nd generation version of the Fuzz-Rite, FY-2, and other units of that era.  That is, where they used a blend pot to shift from a first cleaner gain stage to mostly the output of a second cascaded fuzzier stage, I used a "Tone" pot in more or less the same way. Except that because the first stage overdrive has a very different tone from the second stage, blending in more of the 2nd stage yielded a change in tone as well as a change in intensity; so I labelled it Tone.  Despite the use of op-amps, clipping diodes, and such, like I say, the mindset is essentially the same as an FY-2 or Fuzz-Rite.

[Plexi]

Thanks Mark!

1) I second Jack's suggest changes to the original VL circuit.  In fact, it may be the case that what you have shown is erroneous (a maximum gain of 11x for the clipping stage of an overdrive pedal? ) The only change I would make from his suggestions is that I think he meant for C3 to be 470nf and not 47nf.

Can you be more specific in that part?

[Mark Hammer]

Thanks Mark!

1) I second Jack's suggest changes to the original VL circuit.  In fact, it may be the case that what you have shown is erroneous (a maximum gain of 11x for the clipping stage of an overdrive pedal? ) The only change I would make from his suggestions is that I think he meant for C3 to be 470nf and not 47nf.

Can you be more specific in that part?

Sure.  As shown, R5 is 47k.  With a 470k feedback resistance, that makes for a maximum gain of 11x, which the 500k Gain pot will only lower.  Given that a mild overdrive  like the Tube Screamer has a max gain of 118x, obviously a gain of 11 is not going to get you much grind.  And since the notes suggest lowering R5 to 2k2, I gather the value of 47k is a misprint and should have been 4k7.

As well, subbing a value of 47nf for C3 would actually cut bass, rather than add it.  So I think Jack meant to raise the value of C3 to 470nf (I.e., 0.47uf).  Using 470nf and 2k2 for R5 gets you a maximum gain of  214x with a rolloff around 154hz.  Enough bass for guitar, and enough gain for decent grind.

[amz-fx]

Actually, I meant 47nF (or 0.047uF) along with a 4.7k resistor for R5.

This circuit behaves a bit differently than something like a TS because of where the gain control is located. The highpass cutoff changes with the setting of the pot.

When at low gain settings of 50k to 100k, the freq response is fairly flat across the guitar range.

As you continue to turn up the pot for more gain (resistance goes lower, 50k down to 0R) then the corner frequency starts to climb and you slowly begin to cut some bass, which keeps the sound from getting muddy when you are driving an amp hard. The high pass corner is at its max when gain is at max, and you are getting the most bass cut. It's a nice trick, and the change in tone can sound really good.

Best regards, Jack

[Mark Hammer]

Ahhhhhh.
It was your use of the phrase "Increase C3 to 47nf" that confused me.  I thought your goal was to maintain bass, given that 47nf would be half of the stock 100nf.

Personally, I always thought the role of the ascending bass rolloff in the Distortion+, as gain is increased, was partly to avoid amplifying hum, since it was developed before many of the hum-rejecting pickups we know and love today became widespread.  And I figured the stock cap value in the VL circuit was intended to provide a reasonable amount of bottom at all gain settings.  Not that your suggestion is "wrong" in any sense, but one generally hears more complaints about "It's too thin when I turn up", than "There's too much bass".

Sounds like a job for a C3 toggle! 

regards,
Mark

[amz-fx]

It was your use of the phrase "Increase C3 to 47nf" that confused me. 

Typo on my part. Sorry for the confusion.

Playing at low volumes seems to be more forgiving of extra bass, but when cranked in a loud situation, it is usually better to roll off some of the low end.

The mod that I suggested never gets worse than a Tube Screamer even at max volume, so I expect most players to be okay with it.

Extra switched cap in parallel with C3, or even a a second cap/gain pot combination on a toggle switch is another good mod. 

Best regards, Jack

[Plexi]

Thanks again both for your wise comments.

Thanks Mark!

1) I second Jack's suggest changes to the original VL circuit.  In fact, it may be the case that what you have shown is erroneous (a maximum gain of 11x for the clipping stage of an overdrive pedal? ) The only change I would make from his suggestions is that I think he meant for C3 to be 470nf and not 47nf.

Can you be more specific in that part?

Sure.  As shown, R5 is 47k.  With a 470k feedback resistance, that makes for a maximum gain of 11x, which the 500k Gain pot will only lower.  Given that a mild overdrive  like the Tube Screamer has a max gain of 118x, obviously a gain of 11 is not going to get you much grind.  And since the notes suggest lowering R5 to 2k2, I gather the value of 47k is a misprint and should have been 4k7.

As well, subbing a value of 47nf for C3 would actually cut bass, rather than add it.  So I think Jack meant to raise the value of C3 to 470nf (I.e., 0.47uf).  Using 470nf and 2k2 for R5 gets you a maximum gain of  214x with a rolloff around 154hz.  Enough bass for guitar, and enough gain for decent grind.

Now it's clear like crystal.
My confusion becomes in the "11x" part.

I'll made a search, you're totally right. It should be 4k7.
Any value over to 15k, made almost clean all the way in the "gain" pot.

They placed on R5 the lowest value (2k2 or 4k7) before it oscillate: my test with 50k pot says that 3k6 with loud volumen.. dual op-amp inconsistencies I guess.

BTW, with 4k7 value on R5, the gain pot is smooth and have totally sense now.


So...will be a on-off-on DPDT to C3 cap 

[Mark Hammer]

To avoid audible popping, use two caps in series, and the toggle to shunt/bridge the one or the other..  Two 100nf caps in series will provide an effective cap value of 50nf.  Bridge either one of those two caps, and you're back to 100nf, only without any audible pop, as you would have by using a toggle to add a disconnected cap in parallel.

[Plexi]

To avoid audible popping, use two caps in series, and the toggle to shunt/bridge the one or the other..  Two 100nf caps in series will provide an effective cap value of 50nf.  Bridge either one of those two caps, and you're back to 100nf, only without any audible pop, as you would have by using a toggle to add a disconnected cap in parallel.

Great advise...
And what if I have, for example: 4n7 in the middle (off position), 100nf on one side, and 220nf in another?
104nf/4n7/224nf

[Mark Hammer]

Caps in series are calculated the same way as resistors in parallel.  http://www.electronics-tutorials.ws/capacitor/cap_7.html

If you want a 3-position selection, the optimum strategy is to use 2 different-value caps, such that the centre position of the switch gives you the two in series, or bridges/shunts the one value or the other.  A 100nf and 47nf in series yields 32nf.  So, wiring the common of a 3-way toggle to the junction of two such caps gets you 32nf when the toggle is mid-position (both caps in series), 100nf whenthe 47nf is bridged, and 47nf when the 100nf is bridged.

[Plexi]

Sorry..my bad explanation.
My idea is to use a DPDT switch.
The center position will be always on, cap terminals to each pins of the switch.
Both sides will add in parallel the outher caps.

[Cozybuilder]

Your best bet is to draw out what you have in mind, with all 3 switch positions accounted for. When you see which cap is shorted by flipping the switch, then you remove it from the equation, making the math easy to handle. And this will go a long way in helping you understand the circuitry.

[Plexi]

Of course, Cozy.
That's my idea, and how I've always combined caps:


I use this method with clipping too, leaving the middle lugs empty (No clipping when on-off-on, and two clipping options when using on-on).

Sorry my typo/bad didactics :-)